Forum rules
There are no such things as "stupid" questions. However if you think your question is a bit stupid, then this is the right place for you to post it. Please stick to easy to-the-point questions that you feel people can answer fast. For long and complicated questions prefer the other forums within the support section.
Before you post please read how to get help

Forget about that, it won't help you - this is probably a severe general fault
I suspect you get it for any sudo or gksu (like synaptic) you try to use
Here goes
Check which groups you belong to - 'groups' in a terminal gives you that - and you should find admin among them
Next become root temporarily
Use su in a terminal - you will be asked for your password - your password
Now you are root in that terminal - be careful!
Next the command visudo
Extreme care here because an error in this file creates huge problems as I think you've already noticed
What does that file look like?
Does it have these two lines at the end

# Members of the admin group may gain root privileges
%admin ALL=(ALL) ALL

If not add them - you can copy from here
visudo is a special version of vi so to save and quit you do
:wq +enter
And as stated above - careful